The present invention relates to tunnel kiln apparatus and method for firing ceramic materials (e.g. bricks), whereby, the ceramic materials may be preliminarily cooled after firing without subjecting them to an oxidizing environment (that is, firing and preliminary cooling of the bricks may be accomplished in a reducing atmosphere). As such, the present invention finds particular utility in the production of architectural face-bricks.
Conventionally, ceramic or refractory materials, such as bricks, tiles, and the like (hereinafter noted as bricks for ease of reference) are fired (i.e. cured) in a tunnel kiln whereby the uncured bricks are placed on kiln cars capable of sequential advancement in a train arrangement through the kiln tunnel. The kiln tunnel is itself defined by numerous zones, including a firing zone wherein flue gas from a combustion source is typically introduced into the kiln tunnel at a temperature sufficient to fire the bricks. The bricks are then subsequently cooled, for example, by introduction of ambient cooling air into a zone downstream of the firing zone.
Sometimes it is necessary for a reducing atmosphere to be maintained in the firing zone of the tunnel kiln, as for example, when architectural facing-bricks are produced. However, when firing bricks in a reducing atmosphere, the typical method of cooling the fired bricks by counterflow heat-exchange (i.e. directing cooling air through the tunnel kiln in a direction counter to the conveyance direction of the kiln cars) cannot be utilized. Hence, other techniques must be employed for cooling such bricks fired in a reducing atmosphere.
Reducing atmospheres are usually created in the firing zone of conventional tunnel kilns by the addition of excess fuel at a location towards the end of the firing zone while simultaneously reducing the amount of air flowing from the downstream cooling zone into the firing zone. This reduction of the amount of air flowing from the cooling zone into the firing zone is usually accomplished by reducing the amount of flue gas extracted in the transitional area between the firing and the cooling zones. However, reducing the amount of flue gases extracted from the transitional area between the firing and cooling zones brings about a concomitant required reduction in the amount of cooling air. Obviously, reducing the amount of cooling air will concurrently affect the rate at which the fired bricks are cooled thereby prolonging the bricks' production. Hence, according to conventional practice utilizing tunnel kilns of the type immediately described above, heat is removed from the bricks in the kiln's cooling zone during periodic cooling intervals. That is, the firing of bricks in a reducing atmosphere can only occur for a comparatively short time interval followed by long intervals of oxidation during which the tunnel kiln is operated under normal flow conditions and heat is removed from the bricks in the cooling zone.
Such conventional techniques also do not permit the reducing atmosphere in the firing zone to be contained within precisely defined boundaries since the reduction gases tend to burn up in the transitional area between the firing zone and the cooling zone. This occurrence, in turn, causes the bricks' surfaces to be reoxidized to an extent which cannot be controlled adequately.
According to the present invention, however, ceramic materials, such as bricks, can be fired continually in a reducing atmosphere without subsequent reoxidation occurring thereby resulting in a more consistent production of high quality frost-resistant bricks. In accordance with this invention, method and apparatus are provided whereby, in a tunnel kiln of the type having a firing zone and a downstream cooling zone, the cooling zone being further subdivided so as to establish indirect cooling and direct cooling zones. The indirect and direct cooling zones are partitioned one from the other by a gate seal structure which substantially precludes the oxygen-rich atmosphere of the direct cooling zone (as compared to the oxygen-depleted reducing atmosphere of the indirect cooling zone) from entering the indirect cooling zone. In such a manner, the uncured bricks may be fired and thereafter preliminarily cooled all in a reducing atmosphere.
Preferably, the indirect cooling zone defines a closed-loop cooling path for extracting heated kiln gas therefrom and then reintroducing cooled kiln gases back into the indirect cooling zone. Therefore, the tunnel kiln is capable of firing bricks in a reducing atmosphere within its firing zone without interruption (that is to say on a continual basis) while heat is simultaneously removed from the fired bricks in the cooling zone.
Further aspects and advantages of this invention will become clear after consideration is given to the detailed description thereof which follows.